Electronic x-ray imaging systems, including digital radiography (“DR”) and computed tomography (“CT”) systems, usually employ a reference detector system to monitor the x-ray source. In a direct current (“DC”) x-ray source (conventional x-ray tube) that runs continuously, the reference detector is used to monitor the total output of the source during each sample (a CT view or a DR line) acquired by the x-ray detector during a multi-sample scan. Except for electrical glitches (such as arcs) in the x-ray tube, the variations of x-ray energy output are expected to be relatively slow, and a straightforward x-ray detector mounted close to the source (avoiding interference from whatever object is in the x-ray beam) generally suffices to measure the total energy output of the radiation source. Such a reference detector, however, cannot provide beam-quality information regarding the radiation beam produced by the radiation sources.
As opposed to typical DC x-ray sources, high-energy x-ray sources are typically electron linear accelerators that deliver short-duration pulses of approximately mono energetic electrons to an appropriate target, such as tungsten. Inside the x-ray source, a narrow high-voltage pulse is applied to a high-frequency generator that is coupled to a resonant microwave cavity to accelerate the electrons via very high electric fields. Due to practical limitations of the pulse generator, microwave generator, and cavity, there is some level of uncontrolled variation in both the total energy contained in each pulse of electrons and the effective acceleration voltage (which determines the electrons' kinetic energy) in each pulse. This uncontrolled variation typically increases if the pulse train is not at a constant frequency.
There is a current trend towards material characterization (distinguishing different ranges of atomic number as well as total amounts of material struck by the radiation beam) by comparing x-ray transmission signals at two different energy settings of the x-ray source for a megavolt DR scanner, a scan requiring relative motion between the x-ray system and the object: either the x-ray system moves past a stationary object, or the object moves past a stationary x-ray system. In general, this can be done in one of three ways. First, one source and detector can run two separate scans on the same object where in between the two scans, the energy setting is changed on the single source. Second, two sources, each with its own detector, collect separate images of the same object in one scan of the object. In one example, each of the two detectors may be optimized separately for one of the two sources. Normally, the two imaging systems are separated by a reasonable distance in the direction of travel. Third, one source, capable of rapidly switching between two energy settings, produces an image in one detector where every other line corresponds to one of the two energy settings.
In all cases, the material discrimination is based on comparing the attenuations of the radiation as it passes through the object, as a function of position, for the two source energies. For an x-ray source based on Bremsstrahlung effects, each energy setting for the source determines the source's maximum energy in a broad energy spectrum. For a an x-ray source in the form of a DC tube, the source's maximum energy is determined by the DC voltage applied from a cathode to anode in the tube. For a pulsed accelerator x-ray source, the maximum energy is determined by the relatively narrow range of energy in the accelerated electron beam that hits the target. In many applications, the accuracy or sensitivity of the discrimination depends on the repeatability of these maximum energies or other details of the two spectra.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.